Microalgae are of considerable economic interest because their cells contain molecules of great value, such as medicamental substances, hydrocarbons (glycerol in Dunaliella of salt marshes), proteins which can serve as food for animals or humans, and which can also serve as a substrate for an anaerobic fermentation producing methane, a combustible gas.
In basins for purification of used water, the highest concentrations of microalgae are found which are susceptible of being found in nature (200 to 600 mg/l of dry product with 2.10 algae cells per cubic centimeter). The process of purification is thus accompanied by a large primary production of microalgae estimated as lying between 50 to 150 tons/ha/year. This phytoplankton can be utilized for the production of zooplankton in aquaculture (mainly Rotifera, Copepoda and Daphnia). These animals groups constitute an adequate alimentary chain for feeding young shell fish and fish, but the production of this alimentary chain is under presently known methods costly, sophisticated and difficult to carry out.
As a matter of fact, the most delicate problem encountered in these methods which constitute a form of exploitation of solar energy, is the collecting of microalgae.
The economic difficulties will now be pointed out which come up in this collecting by making reference to the methanization of microphytes which, with respect to the production of renewable energy, have been considered with considerable interest during the past years.
In the most favorable cases--control of the collected species, gathering by methods which require very little power, no drying--the raw energetic content of the production of algae (total quantity of enthalpy used for maintaining the production system in operation, beginning with natural raw material), is about 18 MJ/kg. By methanogene fermentation, 60% of the lower caloric power--a heat gained by combustion with the water formed being in the state of vapor--can be recovered, this caloric power rising to 23.2 MJ/kg, which is 13.9 MJ/kg.
This means, for example, that a mass of 80 kg of microphytes allows the production of 1 GJ by methanization, but that its production having required 1.44 GJ, the total energy balance is negative, because essentially, of the complexity of the separation of microphytes from the aqueous medium.
Because of their dimensions (10-30 .mu.m) and their typical characteristics (living organisms), the microscopic algae pose difficult problems of separation. Certain methods already tested have shown to yield either small results or they consume lots of energy (this is the case of centrifugal decanting or of flotation, tested on a large scale for the separation of plankton).
In the treatment of making water potable, microsifting has been used with success for several years with the primary objective of the elimination of algae. The eliminated suspension materials are recovered in a more concentrated state but in form of a liquid still with a relatively low charge (concentration 20 to 30).
With a filtration on granular material, better results are attained, but at the price of a greater consumption of energy. There again, a recuperation of retained material in suspension can be expected at a rate of concentration of 20 to 30.
The performance of certain of the cited methods can be improved by adding chemical reagents to allow floculation of the algae. But this operation increases the cost of production considerably and may render the use of the product in a food chain impossible.
As a result, if a concentrated suspension of several tens of grams per liter is of any interest, methods such as microsifting or filtering can apply. If a stronger concentration is to be attained, it must be said, that up to this day no method exists which meets both technical and economic requirements.